1. Field of the Invention
The invention relates in general to an automatic sheet feeder and a transmission mechanism thereof, and more particularly to a sheet feeder for an image reading apparatus, such as a scanner, a copier, a printer, a fax machine or a multi-function peripheral, and a transmission mechanism thereof.
2. Description of the Related Art
A typical image reading apparatus has a sheet feeder for automatically transporting originals or sheets. The conventional sheet feeder includes a sheet feeding roller set, a pickup roller for picking up the sheets in a tray, a separation roller for separating the sheets, a transmission mechanism, and a motor for driving the sheet feeding roller set, the pickup roller, the separation roller and the transmission mechanism.
For a duplex document scanner, especially the duplex document scanner with a single image reading head assembly, the motor for driving the roller sets must rotate clockwise and counterclockwise in a duplex scan process to drive the sheet feeding roller set to transport the original in forward and reverse directions and to turn over the original, in order to complete the scanning of two sides of the original.
In order to prevent the pickup roller and the separation roller from being driven by the motor when the fed sheet is being scanned by the image reading head assembly and to prevent other sheets from being fed, a clutch device has to be provided in the sheet feeder to control the pickup roller not to operate corresponding to the clockwise or counterclockwise rotational direction of the separation roller and the motor. However, such mechanism needs another motor to control the clutch device to operate. Thus, the manufacturing cost is higher. In addition, the mechanism has the drawbacks such as large size and heavy weight.
U.S. Patent Publication No. US 2005/0110205 discloses an automatic document feeder using a gear set to replace the clutch device and to control the separation roller to rotate clockwise and counterclockwise. The gear set includes an internal roller with an inner wall formed with a gear structure. A swing arm and a swing gear are mounted inside the internal roller, and a coupling gear and a reverse gear are mounted inside the internal roller. When the swing gear meshes with the reverse gear, the direction of the torque for driving the separation roller may be changed. The drawbacks of using such gear set to replace the combination of the clutch device and the motor are that the more components are used, the combination is complicated, the response speed is low, and the operation is not reliable.